7.
已知动点\(P\)到点\(( \dfrac {1}{2},0)\)的距离比它到直线\(x=- \dfrac {5}{2}\)的距离小\(2\).
\((\)Ⅰ\()\)求动点\(P\)的轨迹方程;
\((\)Ⅱ\()\)记\(P\)点的轨迹为\(E\),过点\(S(2,0)\)斜率为\(k_{1}\)的直线交\(E\)于\(A\),\(B\)两点,\(Q(1,0)\),延长\(AQ\),\(BQ\)与\(E\)交于\(C\),\(D\)两点,设\(CD\)的斜率为\(k_{2}\),证明:\( \dfrac {k_{2}}{k_{1}}\)为定值.